Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such a transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor, and as another material, an oxide semiconductor has been attracting attention.
For example, a semiconductor device that suppresses formation of a parasitic channel due to gate BT stress by using a transistor having a dual gate structure in which an oxide semiconductor film is provided between two gate electrodes is disclosed (see Patent Document 1).